1. Technical Field
The present invention relates to a method for preparing thin films of semiconductors for photovoltaic applications, and more particularly for preparing IB-IIIA-VIA thin films for thin film solar cells.
2. Description of Related Art
Solar cells are sorts of photovoltaic devices converting sunlight to useable electrical power. Because of improvement in conversion efficiency of cells and reduction of costs for manufacturing products in commercial scale, the interest in solar cells has obviously expended in recent years. The most common material applied to the solar cells is silicon, which is in form of a single or polycrystalline thick wafer. However, although the silicon-based solar cells hold the high conversion efficiency at over 20%, a significant level of thickness to absorb the sunlight has been retained so that the decrease of manufacturing cost and the expansion of application on irregular surface are restricted.
Another type of solar cells, namely the “thin-film”, distinguished from the silicon-based solar cells has been developed rapidly due to the lower material cost and the competitive conversion efficiency. The typical structure of a thin-film solar cell essentially includes a substrate, a back contact layer, a p-type semiconductor absorption layer, an n-type junction buffer layer, and a transparent layer. Presently, one of the most potential absorption layers applied in thin-film solar cells uses copper indium diselenide (CuInSe2, CIS) the variants of copper indium gallium diselenide (Cu(In, Ga)Se2, CIGS), or any of these compounds with sulfur replacing the selenium. CIGS or CIS cells have demonstrated the highest efficiency and good stability as comparing with solar cells made from other absorption layer compounds. Sometimes, the acronym CIS and CIGS have been in common, use in literature, so CIGS is used here in an expanded meaning to represent the entire group of CIS based alloys.
To make an absorption layer using CIGS, one of the conventional techniques that yield high-quality CIGS layer for solar cell fabrication is co-evaporation of Cu, In, Ga, and Se onto a heated substrate in a vacuum. Another technique is a two-stage process that after formation of Cu, In, and Ga films on a substrate by means of sputtering or vapor deposition, selenization method under Se or H2Se is reacted with the precursor at elevated temperature. Among them, although the vacuum deposition has an advantage of making high-efficient absorption layer, it shows low materials utilization when making a large-sized absorption layer and also needs expensive equipment. Besides, hydrogen selenide is the most commonly used selenium bearing gas, which is extremely toxic to humans and requires great care in its use.
On account of the disadvantages of the vacuum deposition, methods for formation of CIGS layers using printing processes to coat an ink composition containing a metal oxide mixture particles on a substrate at high temperature are now proposed, which allow one to make a large-sized absorption layer uniform and reduce production costs in manufacturing solar cells. However, because the metal oxide precursor is very stable chemically and thermally to form large crystals, the low efficiency of the absorption layer could be shown.
In addition, regarding the conversion efficiency of CIGS cells, the band gaps of CIGS layer verify continuously from 1.0 (for copper indium selenide) to 1.7 eV (for copper gallium selenide). The band gap can be controlled by altering Ga doping concentrations, and in order to obtain the proper band gap energy, the doping process should be carried out with the compositional ratio of Ga/(In+Ga) ranged from 0.3 to 0.6. If Cu/(In+Ga) ratio is less than 1, a Cu-poor single chalcopyrite phase which has poor performance due to a small grain size is generated. On the other hand, when Cu/(In+Ga) ratio is more than 1, the grain size is increased which results in improved performance. But, in a Cu-rich phase, there are disadvantages that Cu2Se impurities are generated and derive a decrease in the light conversion efficiency caused by higher conductivity of Cu2Se.
There are still some other methods for formation of CIGS materials disclosed. One is that Cu, In, Ga, and Se with low boiling-point amine compounds are reacted in an autoclave under high pressure, which requires higher equipment costs. The other is that complicated chemical compounds are reacted to generate CIGS materials under atmosphere pressure, which requires higher material costs. However, both of the two methods are unsuitable for mass production.